What to do about my dog's aggression?

I have a dachshund whose about 7 years old. When he was younger (like 4 or 3 years) old he got into a fight with a rottweiler at a dog park. Ever since then he's become aggressive towards other dogs. He wasn't neutered then but before the fight he was still fine with dogs. Now he's neutered, we recently got a Cairn/Yorkie terrier mix and he gets along fine with her. He's full sized and she's a bit taller than him. She's scared of other dogs and tends to nip & growl when she feels threatened. He used to only go after males, but now he goes after females, only if they're larger than him. I also wondered if the reason he would get into fights or snap at the dogs is because he could tell I was uneasy. I just wish he wouldn't be so aggressive and would really appreciate the advice.